The present invention provides a method and apparatus for controlling the thickness of an autoclave cured composite component. Specifically, a mold with a cavity having a stop formed about the perimeter of the cavity receives a lay-up, a caul member is placed over the lay-up, and the lay-up is cured causing the caul to move into engagement with the stop. Thus, uniform pressure is applied along the length of the lay-up during the cure cycle resulting in a composite component with precisely controlled thicknesses.
Composite materials are often used in the aerospace industry because of their high strength-to-weight ratio. Aerospace components made from composites must be fabricated such that precise tolerances are maintained. Components that are out of tolerance may adversely affect important characteristics such as structural integrity, performance, fatigue life, etc. Tolerance control is especially important in the manufacture of helicopter flexbeams, for example.
One important dimensional criteria for a flexbeam is the thickness. The thickness varies along the length of the flexbeam. The variation range can be as great as 0.3 inches to 2.5 inches. The flexbeams are typically made from pre-impregnated (prepreg) composite materials of variable per ply thickness. Prepregs are formed from a plurality of fiber members that are saturated with a resinous material. Prepregs are stacked together in various orientations to form laminates.
The fabrication of thick complex prepreg composite components with significant variations in thickness is difficult when using current process technology. Problems such as inconsistent laminate quality and large dimensional variations often occur. This results in increased scrap and reworking of components.
One current composite fabrication method produces autoclave cured components that have good laminate quality but have poor dimensional control. Another fabrication method produces press cured components that have good dimensional control but poor laminate quality. One of the significant factors that causes these difficulties is the variability in the yield or per ply thickness of the prepreg composite materials.
In the past, the preferred method for producing aerospace components has been press-curing where matched metal molds are used. In this process, the prepregs are laid-up in the mold and then press cured according to a specified cure cycle. As discussed above, press curing produces components with suitable dimensional accuracy but with inconsistent laminate quality. An improvement on this process is disclosed in U.S. Pat. No. 5,759,325 (Method For Controlling Thickness During Lay-Up And Fabrication Of Press-Cured Composite Articles), however, even this process produces undesirable variations in complex components. Processing problems are caused because the press mold during cure does not make uniform contact with the lay-up as the mold closes. Initially, only the thickest parts of the lay-up make contact with the moving part of the mold and only these areas receive any process pressure. This is caused by the bulk in the lay-up. Bulk refers to the trapped air that bleeds out of the lay-up or which is compressed as the resin softens during cure. As the cure progresses, more of the surface area comes in contact with the moving section of the mold and the pressure is slowly transferred to the entire length of the lay-up. This variation and movement of pressure in the lay-up during cure is the main problem of press curing. In contrast, autoclave curing can provide uniform pressure to the entire length of the lay-up even with bulk. However, as discussed above, the problem with autoclave curing is dimensional accuracy and repeatability.
Accordingly, it is desirable to have a process that has the benefit of good laminate properties produced by autoclave curing yet also includes the benefit of dimensional accuracy and repeatability produced by press curing.
One of the objects of the present invention is to provide a method and apparatus for precisely controlling the thickness of a composite component in an autoclave environment.
In the preferred embodiment, a fabrication apparatus for producing a composite component having a variable thickness includes a female mold defining a cavity for receiving a lay-up comprised of a plurality of prepreg plies. The mold has a support flange extending into the cavity. A caul is positioned over the lay-up and defines a first axis and a second axis. The caul is made from an anisotropic material having greater flexibility along the one of the axes than the other. Preferably the first axis is a longitudinal axis and the second axis is a lateral axis and the caul is more flexible along the longitudinal axis than the lateral axis such that as the component is cured the caul is drawn into engagement with the support flange. The component is cured in an autoclave that applies a predetermined cure cycle to the component. Preferably, the caul is rigid along the lateral axis and flexible about the longitudinal axis to transmit uniform pressure along the length of the lay-up as the caul is drawn into engagement with the support flange.
Also in the preferred embodiment, the anisotropic material is comprised of a fiber reinforced composite including at least two different laminates. Preferably, a plurality of fiberglass laminates are sandwiched between a plurality of graphite laminates wherein the fiberglass laminates are +/xe2x88x9245 degree laminates and the graphite laminates are 90 degree unidirectional laminates.
In one embodiment, the cavity is defined by a bottom surface, a plurality of walls extending upwardly from the bottom surface, and an open upper face surrounded by a mold edge. The support flange extends into the cavity from each of the walls at a predetermined distance upward from the bottom surface. The predetermined distance varies along the length of the mold such that the support flange conforms to a thickness profile of the composite component to be fabricated.
The method for controlling thickness during fabrication of an autoclave cured component includes the following steps: laying up a plurality of prepreg plies into a female mold defining a cavity including a stop, installing a caul over the lay-up, and curing the lay-up in an autoclave causing the caul to be restrained by the stop. Additional steps include inserting a plurality of deformable members between the cavity walls and lay-up prior placing the caul over the lay-up to apply a back pressure during the cure cycle.
The preferred method includes the steps of vacuum bagging the mold, lay-up, and caul and checking for leaks and heating the mold and the lay-up to a dwell temperature and pressure until the caul is restrained against the stop. Then the temperature and pressure are increased and held for a predetermined amount of time to cure the component.
This unique apparatus provides a method for controlling component thickness during autoclave curing for complex composite components having a large variation in thickness.